Lower olefins, that is, olefins having from 2 to 4 carbon atoms, are particularly suitable starting materials for use in a large number of chemical processes, such as, alkylation, oligomerization and polymerization processes. The preparation of lower olefins from a hydrocarbon feed by thermal cracking of that feed is a well known process and is commercially applied at a large number of petrochemical complexes. Typically, a distillate fraction of a crude oil, commonly a naphtha fraction of a crude oil, is used as a hydrocarbon feed in the thermal cracking process.
For commercial reasons, there is a demand for a thermal cracking process having a high selectivity towards lower olefins and avoiding as much as possible the formation of methane. To minimize the formation of methane, the thermal cracking process is carried out at conditions of relatively low severity. However, a fairly large proportion of the hydrocarbon feed remains uncracked. In contrast, if the thermal cracking process is carried out at relatively high severity conditions, thereby increasing the hydrocarbon conversion, a fairly large proportion of the hydrocarbon feed is cracked to methane. Accordingly, an optimum thermal cracking process would combine a high conversion of the hydrocarbon feed with a high selectivity to lower olefins.
It has been disclosed in European Patent Application No. 161705 (EP 161705) that a fraction of the product of a Fischer-Tropsch synthesis process may be used as a hydrocarbon feed in the thermal cracking process. EP 161705 relates to a process for the preparation of linear C.sub.10 -C.sub.20 olefins, comprising converting, at elevated temperature and pressure and using a specific catalyst, a mixture of carbon monoxide and hydrogen into a mixture of hydrocarbons, substantially consisting of linear paraffins (Fischer-Tropsch product), and converting a heavy fraction of the mixture of hydrocarbons, substantially consisting of C.sub.20.sbsb.- .sup.+ paraffins, into linear C.sub.10 -C.sub.20 olefins by mild thermal cracking. The C.sub.19.sup.- fraction of the mixture of hydrocarbons, substantially consisting of linear paraffins, may be converted into lower olefins by thermal steam cracking. It was found that, when using this C.sub.19.sup.- fraction as a feed for a thermal cracking process, the selectivity towards lower olefins was increased, as compared with a naphtha fraction of a crude oil.